chore: bump version to v0.10.4 Beta and update docs for PR #31 features

Documents CORS_ORIGIN env var, credential preset operator access control,
and server-side bulk device add job in README. Bumps version across all
four locations.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mikrotik-manager-frontend",
-  "version": "0.10.2-beta",
+  "version": "0.10.4-beta",
   "private": true,
   "type": "module",
   "scripts": {